1. Field of the Invention
This invention relates to a silver catalyst to be used in the production of ethylene oxide by catalytic vapor phase oxidation of ethylene with molecular oxygen in the presence of a halogenated inhibitor and to a method for the manufacture of the silver catalyst.
2. Description of Prior Art
The silver catalyst which is used in commercial production of ethylene oxide by catalytic vapor phase oxidation of ethylene with molecular oxygen is required to exhibit high selectivity and high activity in performance, keep high selectivity and high activity for a long period, and suffer only minimal pressure loss in catalyst bed.
Various studies have been made to date for the purpose of improving the catalyst properties and fulfilling the requirements and numerous efforts have been directed to improving carriers, reaction accelerators, and silver compounds. Attempts at improvement of the method of silver deposition are disclosed in Japanese Patent Publication SHO 46(1971)-19,606, U.S. Pat. No. 3,702,259, Japanese Patent Publication No. SHO 59(1984)-29,291, U.S. Pat. No. 4,248,740, U.S. Pat. No. 4,305,844, and U.S. Pat. No. 4,400,308.
As regards reaction accelerators, it has been held that mainly alkali metals and thallium are effective. Various proposals have been made on kinds and amounts of elements and on methods of incorporation as in the specifications of U.S. Pat. No. 3,962,136, U.S. Pat. No. 4,039,561, U.S. Pat. No. 4,207,210, Japanese Patent Laid-open No. SHO 50(1975)-95,213, U.S. Pat. No. 3,962,136, U.S. Pat. No. 4,033,903, U.S. Pat. No. 4,248,740, UK Patent Application No. 2,043,481, and Japanese Patent Laid-oepn No. SHO 56(1981)-5,471, U.S. Pat. No. 4,389,338, for example.
Numerous reports have been proposed on carriers. They appear in the U.S. Pat. No. 3,207,700, Japanese Patent Publication No. SHO 43(1968)-13,137, Japanese Patent Publication No. SHO 45(1970)-21,373, Japanese Patent Publication No. SHO 45(1970)-22,419, Japanese Patent Publication No. SHO 45(1970)-11,217, U.S. Pat. No. 4,368,144, U.S. Pat. No. 2,766,261, U.S. Pat. No. 3,172,893, U.S. Pat. No. 3,664,970, and U.S. Pat. No. 4,242,235. Many of these reports concern pore size distribution and specific surface area of carrier.
Further, U.S. Pat. No. 3,664,970 discloses saddles as the carrier, but the catalyst prepared by using such carrier is appropriate for unprovid oxidation of ethylene in the absence of halogenated inhibitor.
All these specifications make no mention of improvement in selectivity of catalysis or pressure performance of catalyst bed due to selection of the shape of carrier but merely go the length of disclosing pellets, spheres, and Raschig rings, i.e. the shapes of catalysts preponderantly adopted to date on commercial scale.
U.S. Pat. No. 4,389,338 discloses a method for the preparation of a silver catalyst by the steps of depositing silver and an alkali metal and/or thallium as reaction accelerators on a porous inorganic refractory carrier and subjecting the resultant composite to a high temperature treatment in an inert gas having a final oxygen content of not more than 3% by volume at elevated temperatures in the range of 550.degree. to 950.degree. C.
In the specification of U.S. Pat. No. 4,248,740 is disclosed a method which produces a silver catalyst by impregnating a porous inorganic refractory carrier with a silver compound solution containing a reducing compound, subjecting the impregnated carrier to a heat treatment for reduction thereby causing metallic silver to be dispersed and deposited on the surface of the carrier and on the inner surface of fine pores in the carrier, then washing the composite with water and/or lower alcohol, drying the washed composite, impregnating the dried composite with a solution containing an alkali metal and/or alkali metal compound, and drying the impregnated composite through vaporization of the liquid component. These are examples of the methods for the production of commercially available silver catalysts possessed of catalyst properties combining the highest selectivity, the highest activity, and keep the highest selectivity and the highest activity for the longest period. They, however, are not fully satisfactory in selectivity.
Concerning carriers for silver catalysts used in the production of ethylene oxide, many points remain yet to be elucidated and many problems remain yet to be solved. For example, physical properties such as combination of components for carrier, specific surface area, pore diameter, pore distribution, pore volume, porosity, grain size, and shape of carrier and chemical properties possessed by such carrier materials as .alpha.-alumina, silicon carbide, silica, and zirconia await improvement for optimization.
An object of this invention, therefore, is to provide a novel silver catalyst for the proudction of ethylene oxide which catalyst is used in the production of ethylene oxide by catalytic vapor phase oxidation of ethylene with molecular oxygen in the presence of a halogenated inhibitor and a method for the manufacture of the silver catalyst.
Another object of this invention is to provide a silver catalyst which permits production of ethylene oxide from ethylene and molecular oxygen in the presence of a halogenated inhibitor with high selectivity and which suffers only minimal pressure loss in catalyst bed and a method for the manufacture of the silver catalyst.